At present, a multi-antenna technology at physical layer has become one of crucial technologies in a next-generation wireless communication system. The multi-antenna technology has numerous advantages, for example, the throughput of the system can be improved with a multiplexing gain of multiple antennas, the performance of the system can be improved with a diversity gain of the multiple antennas, receiving terminals can be distinguished from each other with a directivity gain of the antennas to remove interference between the receiving terminals, etc.
A Multiple-Input Multiple-Output (MIMO) system uses multiple transmitting and receiving antennas and utilizes joint processing of a signal in the space, time and frequency domains to obtain rate, diversity and array gains. In the MIMO system, if a transmitter can acquire channel state information somehow, a transmission signal can be optimized dependent upon a channel characteristic to improve a reception quality and lower required complexity of a receiver. In a real system, typically a feedback of quantized channel state information is adopted to lower feedback overhead and feedback delay. The channel state information can be quantized in terms of a channel matrix and a statistic thereof or of a pre-processing parameter recommended by the receiver for use. The quantized channel state information is mapped into a set of a limited number of elements, which is referred to as a codebook.
In a codebook-based closed-loop MIMO system, a codebook is a key factor influencing the performance of the system. In each of the transmission modes 4, 5 and 6 of the Rel-8 long Term Evolution (LTE), a Precoding Matrix Indicator (PMI) feedback mechanism is adopted, that is, a User Equipment (UE) measures a channel according to a common pilot and reports index information of a codeword that it recommends for use to an eNodeB (eNB). With four common pilot ports, for example, a Householder transformation-based codebook is adopted, and there are sixteen selectable pre-coding matrices for each Rank of the closed-loop spatially multiplexing mode, i.e., the transmission mode 4. General characteristics of an Rel-8 LTE 4-antenna codebook include: 1) a Rank1 codebook is obtained by searching while considering a tradeoff between uncorrelated and correlated channels; 2) a nesting relationship holds between codebooks at the other Ranks and the Rank1 codebook; 3) a 8PSK (Phase Shift Keying) element is adopted for each codebook element to lower calculation complexity; 4) there are constant modulus and unitary characteristics; and 5) complexity of selecting a codeword can be lowered with a characteristic of the Householder transformation.
In an MIMO-based wireless communication system, e.g., an Rd-10 LTE-A system, the use of the PMI feedback mechanism can be continued, but the number of configuration combinations of Ranks and the numbers of transmitting antenna ports is far larger than that in the Rel-8 LTE due to an increased number (possibly eight, for example) of transmitting antennas. On one hand, no codebook has been defined for a large number of configuration combinations of Ranks and the number of transmitting antenna ports. On the other hand, the necessity to design a codebook for a spatially correlated channel has become more prominent as compared with the Rel-8 LTE system because the number of antennas constantly increases and the size of a base station is limited so that it is inevitably necessary to shorten the spacing between antennas and/or adopt polarized antennas. In view of the foregoing considerations, a linear codebook for a spatially correlated channel and a designing method thereof are indispensable technology issues in future wireless communication system researches and standardization. Furthermore, researchers consider as much as possible reserving the characteristics of the Rel-8 LTE codebook while designing a codebook in view of the good performance and excellent characteristics of the Rel-8 LTE 4-antenna codebook.
The existing Householder transformation method can not be applied directly to designing of an 8-antenna codebook. Generally the following several codebooks can inherit the characteristics (or a part thereof) of the Rel-8 LTE codebook at present.
A codebook 1: The Rel-8 codebook is transformed into an 8-antenna array codebook at the Rank1 The codebook 1 has a drawback of failing to satisfy a nesting relationship although the codebook 1 inherits a part of the characteristics of the Rel-8 LTE 4-antenna codebook, i.e., the constant modulus and the use of an 8PSK element. Since the codebook 1 is based upon extending the Rel-8 LTE 4-antenna codebook which results from optimization in view of both spatially correlated and spatially uncorrelated channels and a part of codewords of which is not suitable for the spatially correlated channel, there is a performance loss of the codebook 1 for an array of antennas at a small spacing.
A codebook 2: A number 47 of 8×8 complex matrices with their elements being 8PSK elements are constructed in complex Hadamard transformation, and then a Rank1 codebook is obtained by searching in the optimizing method with a tradeoff between uncorrelated and correlated channels. The codebook 2 has a drawback of adopting only 8PSK elements although the codebook 2 inherits most of the characteristics of the Rel-8 LTE codebook, i.e., the constant modulus, unitary and nesting characteristics. Since a space consists of a number 47 of 8×8 complex matrices with their elements being 8PSK elements is searched for the codebook 2 in a search criterion considering a performance tradeoff between spatially correlated and spatially uncorrelated channels, the performance of the codebook 2 over the spatially correlated channel is degraded.
A codebook 3: The Rank codebook is derived by multiplying sixteen different diagonal matrices by a 8-DFT (Discrete Fourier Transform) matrix, and the Rank1 codebook is composed of first columns of the sixteen 8×8 complex matrices. The codebook 3 has such a drawback that although the codebook 3 inherits a part of the characteristics of the Rel-8 LTE codebook, i.e., the constant modulus, unitary and nesting characteristics, complexity of pre-coding calculation will be increased because 16PSK elements are adopted. If there is no significant performance gain over other codebooks, the increased complexity of pre-coding calculation will become an extra burden.
A codebook 4: the codebook consists of respective rows (or columns) of one 8-DFT matrix and one 8×8 unitary matrix. The codebook 4 has a drawback of adopting only 8PSK elements although the codebook 4 inherits a part of the characteristics of the Rel-8 LTE codebook, i.e., the constant modulus, unitary and nesting characteristics. Also since the DFT matrix complies with the construction of an array response, both the codebook 3 and the codebook 4 are suitable for a pre-coding MIMO system over a spatially correlated channel. However, the angles of corresponding array responses of respective codewords in the codebook 3 and the codebook 4 are unchanged regardless of a specific distribution characteristic of the channel, so a performance loss of the pre-coding MIMO system may be incurred in a specific scenario.
Therefore, it is desired to propose an efficient design of a codebook and a solution to information transmission and reception using the codebook.